1. Field of the Invention
This invention relates to a very light-weighted axial-flow fan apparatus suitably used for various machines and devices.
2. Description of the Prior Art
Normally, an axial-flow fan apparatus includes a single-phase energized brushless motor (where it includes a two phase arrangement of armature coils, it is sometimes called a two phase motor, but it is accurate to call it a single-phase motor due to its energizing method) installed in a motor casing located in a body of the fan apparatus.
Thus, as various machines and devices such as business machines have been developed, axial-flow fan apparatus are required to be very light-weight, very inexpensive, very small and very flattened.
One axial-flow fan apparatus which meets such requirements is an axial-flow fan apparatus in which a flattened single-phase energized coreless brushless motor which necessitates only one position detecting element is installed in a motor casing of the apparatus.
Such a flattened single-phase energized coreless brushless motor can be very thin and light-weight due to its coreless, flattened configuration and can be very inexpensive because it includes only one position detecting element and a single-phase energized controlling circuit and hence its energization controlling circuit is simple.
In a single-phase brushless motor of the type, there is a so-called dead point at which energization to armature coils is switched and no torque (starting torque) is generated by the motor. Thus, such a single-phase brushless motor has a drawback that it cannot start itself if a rotor field magnet of the motor is just at a dead point upon starting of the motor. Therefore, a single-phase energized brushless motor is normally provided with a cogging generating magnetic member (an iron piece or a magnet) cooperating with a field magnet for generating a cogging torque in addition to an armature torque generated by an armature coil and the field magnet in order to prevent the torque at such dead points from reducing to zero to allow self-starting of the motor.
This applies not only to a single-phase brushless motor of the cored type but also to a single-phase brushless motor of the coreless type.
Here, it is desirable to employ a flattened single-phase energized coreless brushless motor in order to obtain an inexpensive, light-weight, very flattened axial-flow fan apparatus, and in order to obtain a more inexpensive, more light-weight, more flattened axial-flow fan apparatus, it is desirable to employ a flattened coreless brushless motor which includes no stator yoke therein.
However, where a flattened single-phase coreless brushless motor which includes no stator yoke therein is employed in an axial-flow fan, apparatus magnetic flux from the magnet rotor (field magnet) will leak axially (toward a bottom of a body of the axial-flow fan apparatus) due to the absence of a stator yoke. Accordingly, if such an axial-flow fan apparatus is used for a particular apparatus for an object such as cooling, there is the possibility that such leakage of flux will have a bad influence on apparatus or devices around the apparatus. This phenomenon readily appears particularly where the body (particularly a motor casing) of the axial-flow fan apparatus is made as a non-magnetic member. Leakage of a magnetic flux appears frequently where a single-phase energized brushless motor employed in an axial-flow fan apparatus is of flattened coreless brushless type.
In order to prevent such leakage of magnetic flux, a body or a motor casing of an axial-flow fan apparatus may be made of a magnetic material. However, this will make the axial-flow fan apparatus heavy, and it will be very inconvenient to incorporate it in a housing of a particular type. To the contrary, in order to minimize the weight and further reduce the cost, a body and a motor casing of an axial-flow fan apparatus may desirably be made integrally of a resin material. Further, it is desirable to omit a stator yoke. However, this will lead to a drawback that leakage of a magnetic flux will appear.